Long Term Evolution or LTE uses Orthogonal Frequency Division Multiplexing or OFDM in the downlink and DFT-spread OFDM in the uplink, where DFT denotes “Discrete Fourier Transform”. The basic LTE physical resources can thus be seen as a time-frequency grid as illustrated in FIG. 1, where each resource element corresponds to one subcarrier during one OFDM symbol interval on a particular antenna port. An antenna port is defined such that the channel over which a symbol on the antenna port is conveyed can be inferred from the channel over which another symbol on the same antenna port is conveyed. There is one resource grid per antenna port, and resource elements may be referred to as “Resource Elements” or REs.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 milliseconds. Each radio frame includes ten equally sized subframes of 1 millisecond. FIG. 2 illustrates this arrangement and one sees from the diagram that each subframe is divided into two slots, with each slot having a duration of 0.5 milliseconds.
Resource allocation in LTE is described in terms of Physical Resource Blocks or PRBs. As shown in FIG. 3, a PRB corresponds to one slot in the time domain and 12 contiguous 15 kHz subcarriers in the frequency domain. The bandwidth, NBW, of the overall system determines the number of PRBs in each slot and each PRB spans 7 or 6 OFDM symbols, depending upon the length of the Cyclic Prefix, CP, used. Two consecutive PRBs in time represent a PRB pair, and correspond to the time interval upon which user scheduling operates.
In that regard, transmissions in LTE are dynamically scheduled based on transmitting downlink assignments and uplink grants to targeted UEs. According to 3GPP Release 8, such control information is transmitted in a defined control region using Physical Downlink Control Channels or PDCCHs targeted to specific UEs. See “Universal Mobile Telecommunications System (UMTS); Technical Specifications and Technical Reports for a UTRAN-based 3GPP system”, 3GPP TS 21.101 version 8.4.0 Release 8. The search space for PDCCH reception is known to the UEs, which blindly decode those portions of the received signal to find PDCCHs targeted to them. More broadly, PDCCHs are used to convey UE-specific scheduling assignments for the downlink and uplink grants, as noted, and is further used for Physical Random Access Channel or PRACH responses, uplink power control commands, and common scheduling assignments for signaling messages that among other things include system information, paging.
FIG. 4 illustrates that a “normal” downlink subframe includes a control region at the beginning of the subframe, followed by a data region. The size of the control region in which PDCCHs are transmitted can vary in size from one to four OFDM symbols in dependence on the involved configuration. A Physical Control Format Indicator or PCFICH is used to indicate the control region length and is transmitted within the control region at locations known by the UEs. A UE thus learns the size of the control region in a given downlink subframe by decoding the PCFICH transmitted in that subframe, and therefore knows in which OFDM symbol the data transmission starts.
To better understand the construction of PDCCHs, consider that the term “CCE” denotes Control Channel Elements and that each CCE consists of nine REGs, where the term “REG” denotes a Resource Element Group. Each REG consists of four REs. LTE defines four PDCCH formats 0-3, which use aggregation levels of 1, 2, 4, and 8 CCEs, respectively. Given the modulation format used for PDCCH transmission, two bits can be transmitted on a single RE aggregated within a PDCCH, therefore, with 1 CCE=9 REGs=4 REs/REG×9 REGs=36 REs×2 bits/symbol, one can transmit 72 bits via a format 0 PDCCH, 144 bits via a format 1 PDCCH, etc. As noted, PDCCHs are transmitted in the defined control region—the first 1-4 symbols—of any given downlink subframe and extend over substantially the entire system. Thus, the size of the control region in the given downlink subframe and the overall system bandwidth define the number of overall CCEs available for PDCCH transmission.
FIG. 4 also illustrates the presence of Cell-specific Reference Symbols or CRS within the downlink subframe. The CRS location and modulation sequence are known by the UEs and used for estimation of the radio channel and in turn the channel estimates are used in the demodulation of data by the UEs. CRS are transmitted in all downlink subframes and in addition to assisting downlink channel estimation they are also used for mobility measurements performed by the UEs. Because the CRS are common to all UEs in a cell, the transmission of CRS cannot be easily adapted to suit the needs of a particular UE. Therefore, LTE also supports UE-specific RS aimed only for assisting channel estimation for demodulation purposes. These UE-specific RS are referred to as Demodulation Reference Symbols or DMRS. DMRS for a particular UE are placed in the data region of the downlink subframe, as part of Physical Downlink Shared Channel, PDSCH, transmissions.
Release 11 by the 3GPP introduced enhanced PDCCHs, or EPDCCHs. An EPDCCH uses REs in the data region associated with PDSCH transmissions, rather than REs within the defined control region at the beginning of the subframe. See “Universal Mobile Telecommunications System (UMTS); Technical Specifications and Technical Reports for a UTRAN-based 3GPP system”, 3GPP TS 21.101 version 11.0.0 Release 11.
FIG. 5 provides a basic illustration of PRB pairs allocated from the data region of a downlink subframe, for use in the transmission of given EPDCCHs. The remaining PRB pairs in the data portion of the subframe can be used for PDSCH transmissions; hence the EPDCCH transmissions are frequency multiplexed with PDSCH transmissions. That arrangement is contrary to PDCCH transmissions, which are time multiplexed with respect to PDSCH transmissions—i.e., PDCCH transmission occur only in the control portion of the downlink subframe, which occurs in time before the data portion in which PDSCH transmissions are performed. Notably, Release 11 does not support multiplexing of PDSCH and any EPDCCH transmission within the same PRB pair.
According to current convention, a UE can be configured with one or two EPDCCH sets of PRB pairs. Each set consists of N=2, 4, or 8 PRB pairs. Furthermore, two modes of EPDCCH transmission are supported, the localized and the distributed EPDCCH transmission, and each set is independently configured as being of localized or distributed type. In distributed transmission, an EPDCCH is mapped to resource elements in an EPDCCH set in a distributed manner (across all PRB pairs in the set). In this way frequency diversity can be achieved for the EPDCCH message. See FIG. 6 for an example distributed transmission, where N=4 is illustrated.
In a localized transmission, an EPDCCH is mapped to one PRB pair only, if the space allows, which is always possible for aggregation levels one and two, and also for aggregation level four, for the case of a normal, “unconstrained” subframe and a normal CP length. In case the aggregation level of the EPDCCH is too large so that it does not fit into one PRB pair, a second PRB pair is used as well, and so on, using more PRB pairs, until all ECCE belonging to the EPDCCH have been mapped. See FIG. 7 for an illustration of localized transmission. Here, an “unconstrained” or normal subframe is one having a PDSCH or data region that is not abbreviated, such as in the case of “special” subframes in TDD LTE that include uplink and downlink portions, or is not given over to another purpose, such as Multicast-Broadcast Single Frequency Network, MBSFN, transmissions.
In the EPDCCH context, CCEs are referred to as ECCEs and REGs are referred to as EREGs. To facilitate the mapping of ECCEs, to physical resources, each PRB pair allocated for EPDCCH use is divided into 16 EREGs and each ECCE is split into L=4 or L=8 EREGs for normal and extended CP, respectively. An EPDCCH is consequently mapped to a multiple of four or eight EREGs. The particular multiple used depends on the aggregation level of the EPDCCH. Further, the EREGs belonging to an EPDCCH reside in either a single PRB pair, which is typical for localized transmission, or a multiple of PRB pairs, which is typical for distributed transmission.
The division of a PRB pair with normal CP into EREGs is illustrated in FIG. 8 for an unconstrained subframe. Each block or tile is an RE and the tile number corresponds to the EREG the RE is grouped within. For example, tiles having the dotted background all belong to same EREG indexed at 0. Furthermore, the L=4 or L=8 EREGs respectively are grouped into the ECCEs as specified in 3GPP Technical Specification 36.213. For example, ECCE#0 is for L=4 using EREG#{0,4,8,12} and ECCE#1 is using EREG#{4,8,12,0} etc. Furthermore, ECCE#4 is using EREG#{1,5,9,13} and so on. For normal, unconstrained subframes, an EPDCCH set of N PRB pairs has a total of 4N ECCEs for L=4 and 2N ECCEs for L=8. Note that this grouping of REs into EREGs is the same for both localized and distributed EPDCCH sets. In localized sets, the EREG belonging to an ECCE (e.g. EREG#0,4,8,12) is taken from the same PRB pair, while in distributed EPDCCH transmission and for, e.g., N=4, the four EREGs belonging to an ECCE is taken from the four different PRB pairs respectively.
When mapping an EPDCCH to REs, if the EPDCCH collides with the use of REs by other signals, such as own-cell CRS or the own-cell legacy control region, the other signals have priority. Therefore, the EPDCCH is mapped around any such “occupied” REs and code chain rate matching is applied. This approach means that the effective number of available REs per EREG for EPDCCH use is usually less than the 9 REs. However, the colliding signals do not introduce interference into EPDCCH decoding at the UEs because EPDCCH is mapped around them. The UE normally monitors a set of ECCE aggregation levels {1, 2, 4, 8} but if the number of available REs per PRB pair is less than a threshold, X_thresh=104, then the UE instead monitors a shifted set of aggregation levels {2, 4, 8, 16}, because the smallest aggregation level, AL=1, has too few available REs to be practically useful.
The reception of EPDCCH at the UEs relies on DMRS for demodulation, where antenna ports 107-110 have been defined for this purpose. These antenna ports are the same as ports 7-10 used for PDSCH demodulation, apart from an independent DMRS scrambling sequence initialization. In a localized transmission case, and with normal CP, all four antenna ports are available and the EPDCCH use one of these in the PRB pair. In distributed transmission, two of the four antenna ports are used for EPDCCH demodulation in the PRB pair, so as to achieve spatial diversity for the EPDCCH transmission. In normal CP, ports 107 and 109 are used, corresponding to ports 7 and 9 for PDSCH. With extended CP, ports 107 and 108 are used for EPDCCH.
As noted, according to current convention, a UE can be configured with one or two EPDCCH sets of PRB pairs. In accordance with the convention defined in the applicable standards, each set consists of N={2,4,8} PRB pairs. A normal or unconstrained subframe has a nominal number of REs available for EPDCCH use in each PRB pair within the base set. However, with this conventional definition of PRB pair sets, EPDCCH capacity and coverage becomes problematic in “constrained” subframes, which, in comparison to a normal or unconstrained subframe, have fewer numbers of REs available for EPDCCH use within the underlying PRB pairs used to form the base sets of N=2, 4, or 8 PRB pairs allocated for EPDCCH use.
That is, for constrained subframes, the PRB pairs for EPDCCH use have a fewer number of REs available within them, as compared to the number available in a normal, unconstrained subframe. Consider an example where in a constrained subframe 16 EREGs are available for EPDCCH allocation, but each EREG includes only one or two allocable REs per PRB pair. One ECCE therefore will contain between N and 2N REs, when N represents the number of PRB pairs in the base set of PRB pairs allocated for EPDCCH use in the subframe. For N=8, the maximum number of REs per ECCE would equal 16. Hence, a large number of ECCEs would need to be aggregated to provide the same EPDCCH coverage or capacity as offered in normal subframes. A similar problem holds for essentially any type of shortened or special subframe, all of which are broadly referred to herein as constrained subframes.
FIG. 9 provides an example of the above circumstances in the context of a special subframe as used in TDD LTE. One sees a sequence or series of TDD subframes, including normal downlink, “D”, and uplink, “U”, subframes that are entirely allocated to downlink and uplink use, respectively. The illustrated special subframe is interposed between the normal downlink and uplink subframes. The special subframe includes a downlink portion and an uplink portion, with the two portions separated by a guard time. Because the special subframe accommodates downlink and uplink portions, the downlink portion of a special subframe is abbreviated as compared to a normal downlink subframe and the available number of REs within an ECCE is therefore abbreviated as compared to a normal downlink subframe.
The specific structure of special subframes in TDD LTE depends on the special subframe configuration in use, of which LTE defines nine configurations. However, in general, transmissions that rely on DMRS cannot be present in all special subframes because the guard band and uplink portion of special subframes overlap the DMRS transmission patterns. Hence, UEs are not expected to receive PDSCH in a special subframe with special subframe configuration 0 or 5 in normal CP, or special subframe configuration 0, 4, or 7 in extended CP. Furthermore, the UE is not expected to receive PDSCH in a special subframe with special subframe configuration 0 or 5 in normal CP, or special subframe configuration 0, 4 in extended CP. In such instances, EPDCCH is not used for scheduling and PDCCH is used instead.
As recognized herein, however, reliance on PDCCH in such instances is problematic. For example, from Release 11 forward, new carrier types are being considered. One type of carrier, noted here as Type A, retains backwards compatibility with Release 8 UEs. Consequently, this carrier type also may be referred to as a Legacy Carrier Type or LCT. The second type of carrier may be identified as a Type B or New Carrier Type, NCT. The NCT is described as a carrier type that contains either no CRS at all or many fewer CRS in frequency and/or in time. Further, the NCT does not contain any PDCCH but does contain EPDCCH. As noted, an EPDCCH does not rely on CRS for demodulation. While the NCT is attractive for its energy efficiency properties, its low control and reference signal overhead and low level of interference generation in networks, as compared to the LCT, it does not allow for the use of PDCCH as a fallback mechanism for transmitting user scheduling information in constrained subframes where DMRS/EPDCCH cannot be transmitted according to current conventions. Because there may be a large number of problematic, constrained subframes present in the context of NCT usage, there may be significant throughput reduction arising from the inability to send scheduling information in constrained subframes.
Multicast/Broadcast Single Frequency Network, MBSFN, subframes represent another noted example of constrained subframes. FIG. 10 illustrates an MBSFN subframe. As indicated by the name, the original intention for MBSFN subframes was to support multicast such as Multimedia broadcast services. An MBSFN subframe consists of a control region of length one or two OFDM symbols, which is in essence identical to the control-region counterpart in a normal subframe. However, in an MBSFN subframe, the control region is followed by an MBSFN region. Retention of the control region in MBSFN subframes provides for the transmission of control signaling using PDCCH, which is necessary for scheduling uplink transmissions. All terminals, from LTE release 8 and onwards, are capable of receiving the control region of an MBSFN subframe. Terminals not capable of receiving transmissions in the MBSFN region will simply ignore those transmissions.
Information about the set of subframes that are configured as MBSFN subframes in a cell is provided as part of the transmitted system information. In principle, an arbitrary pattern of MBSFN subframes can be configured with the pattern repeating after 40 milliseconds. However, as information necessary to operate the system needs to be transmitted in order for UEs to find and connect to a cell, subframes where such information is provided cannot be configured as MBSFN subframes. Therefore, subframes 0, 4, 5, and 9 for Frequency Division Duplex, FDD, and subframes 0, 1, 5, and 6 for TDD cannot be configured as MBSFN subframes. That leaves the remaining six subframes as candidates for possible configuration as MBSFN subframes.
One possible solution to addressing EPDCCH resource shortages in constrained subframes, such as MBSFN subframes or special subframes used in TDD LTE, depends on simply increasing the value of N for constrained subframes as compared to unconstrained subframes. That is, one may increase the number of PRB pairs included in the base sets of PRB pairs allocated for EPDCCH usage. However, it is recognized herein that considerable complexity surrounds any solution to make more REs available for EPDCCH usage in a constrained subframe. For example, increasing the value of N for constrained subframes would require new definitions of how an ECCE is defined in terms of groupings of EREGs. If the number of PRB pairs included in the base set of PRB pairs is increased to N=32, for example, then an ECCE would consist of a list of 32 EREGs, which is inflexible. Further, supporting new values of N for EPDCCH sets would require specification changes in terms of search space definitions and blind decoding candidate distributions, for proper UE operation.